Undercut refers to the convex or concave area of part. When removing the part from the mold, you need some device so that the part of the mold which contacts to the undercut can be moved to remove the part from the mold since the part will not be removable directly without the device. This process is called Undercut process. Undercut process uses different methods depending upon whether the undercut is outside or inside the part. Here are some examples of parts with undercut. Push-pinGrip of a driverGrip of a cupNow, lets learn about external slide core method, internal slide method, and hydraulic cylinder method of undercut process.External Slide Core Method The external slide core method is usually used for the process of outer undercut. Working along with the open / close movement of a mold, this method slides the slide core to process the undercut. The hydraulic cylinder method is also available, which will be explained in detail laterSlide Core Body UnitThe portion where the undercut process is performed linked along with the movement of mold opening. The amount of movement of the slide core and the angle and the length of the angular pin are key factors to this unit. The locking block installed to the cavity plate receives the pressure from the injection when closing the mold. Upon opening of the mold, the slide core moves backward by the force from the angular pin and the spring, thus releasing the part.As the hole will be misaligned and the angular pin will not be inserted if the slide core moves too far backward, a core stop block needs to be installed. This correlation is illustrated below.(Click each name in the figure to display the description.) The relief distance and the tilt angle of the angular pin are as follows: S: Undercut amount S1: Slide stroke amount : Tilt angle of slide core : Tilt angle of angular pin S1 = S + 5mm up =+(2 - 5 ) 20 In order to prevent the slide core and the loking block from intervening with each other in mold opening and closing, make the tilt angle of the angular pin smaller than that of the locking block. Also, a gap of 0.5mm is added to the hole for angular pin for errors occurring at processing.Push Back SpringThe mass when the slide core is sliding varies depending on the direction of sliding.Suppose you install a 5kg slide core on the side parallel to the ground. How long should the spring be? The stroke is 20mm. D: Spring diameter L: Spring free length (see a catalog by each manufacturer) E: Spring close length (see a catalog by each manufacturer) F: Stroke G: Length of shrinkage by slide weight K: Spring constant (see a catalog by each manufacturer) W: Slide weight If using light load K = 0.6kgf / mm for spring: Spring constant (K)Free length (L)Max. deflection (F + G)Outer diameter (D)Usage count0.6kg/mm65mm32.5mm16mm350,000 timesTo figure out the slide mass, it is 1.5 times the weight of the slide core: W =5*1.5=7.5kgAs the length of shrinkage due to the weight of the slide mass is: G G = W W/K K : G G = 7.5/0.6 = 12.5mmAs the free length of the spring is: L L = E E + F F + G G : L L = 32.5 + 20 + 12.5 = 65. Referring to the catalog, Use the spring with L L = 65.Answer D D = 16 L L = 65mm CoreCore PlatePlate / / CavityCavity PlatePlateA steel device to mold a molding material into a certain shape. The section that is engaged in opening / closing movement is called a core plate, and the section that is not is called a cavity plate. Generally, the front side of a part is a cavity plate and the rear side is a core plate. The core plate leaves a trace on a part, because it has an ejector pin to push the part. The cavity plate has a sprue bush, which is the entrance for molten plastic.LockingLocking BlockBlockA block to push a slide core that is ejected back by injection pressure.Angle of Angular Pin and Angle of Angled Side of Locking BlockAngle of angular pin: Angle of side of locking block: (1) If :If the angle of an angular pin is larger than the angled side of a locking pin, the tilted sides of a slide core and the locking block may collide with each other, possibly damaging the mold.(2) If = : If the angle of an angular pin and a tilt angle of a locking block are equal, the slide core cannot be lowered, since it is pushed down by a spring, which puts the locking block over the slide core for the clearance of the angled hole of the slide core.(3) If : Since the angled sides of the slide core and the locking block touch each other only when the mold is completely closed, the movement is smooth.From (1), (2), and (3), the angle of the angular pin must be smaller than the angle of the side. The normal angle of the angular pin is 12 - 25 degree, and the angle of the parting side is basically larger than the pin angle.CoreCore StopStop BlockBlockA part that determines the position where a slide core retreats when opening the mold. In addition to the core stop block